When insulating paper in an insulating oil deteriorates, the decrease in the mechanical strength thereof is quite large, while the decrease in the dielectric breakdown voltage is small. When external short-circuiting occurs in an oil-filled electric apparatus such as an oil-filled transformer, oil-filled reactor or the like, a large electromagnetic mechanical force is generated inside the electric apparatus. Accordingly, from the stand point of the lowered mechanical strength, the life of the electric apparatus has been estimated from the degree of deterioration of the insulating paper used to insulate conductive material.
The degree of deterioration of the insulating paper is evaluated by measuring its tensile strength the degree of polymerization and the like, or by observing the deterioration speed. Evaluating the degree of deterioration of the insulating paper by determining the amount of a component such as furfural, acetoaldehyde, acetone, carbon dioxide, carbon monoxide and the like which is generated by deterioration of the insulating paper is another method currently being studied.
These methods of evaluating the deterioration of insulating paper are carried out by an accelerated deterioration test at an elevated temperature of over a hundred ten degrees centigrade to two hundred degrees centigrade. Generally, an oil-filled electric apparatus is operated at a temperature of not more than 110.degree. C., and the degree of deterioration of the insulating paper is predicted by extrapolating the accelerated deterioration test data obtained at the elevated temperature at the operating temperature of the oil-filled electric apparatus (typically up to 110.degree. C.).
The above-mentioned method in which the high temperature accelerated deterioration test data is extrapolated to 110.degree. C. or less to estimate the degree of the deterioration of the insulating paper presents the following problem.
Water (H.sub.2 O) and methane (CH.sub.4) have the same molecular weight, 18, but the boiling points thereof are very different; that of water is 100.degree. C. and that of methane is -164.degree. C. The difference in the boiling points is felt to result from the hydrogen bond formed in the water molecule. The material of the insulating paper is typically cellulose, which has many hydroxyl groups (--OH) and also since oxygen is an atom having a large electronegativity, that is the capacity for attracting electrons of neighbouring atoms to which the oxygen is bonded, the electron of the hydrogen atom is attracted, the oxygen itself is negatively polarized and the hydrogen is positively polarized. Hydrogen and oxygen also approach each other and hydroxyl groups attract each other to form a physical hydrogen bond. As is shown by the difference in the boiling point of water and methane, the hydrogen bond has a large effect on thermal characteristics. One can expect that a similar phenomenon also occurs in the insulating paper the raw material thereof being cellulose having many hydroxyl groups. That is, one can expect that the mechanism of deterioration of the insulating paper also changes at around 100.degree. C. due to the intervening hydrogen bond.
Accordingly, it became clear that the above-mentioned conventional evaluation method, in which the accelerated deterioration test data obtained at an elevated temperature is extrapolated to the operating temperature of the electric apparatus which is not more than 110.degree. C., has a problem in precision.
Even with the high temperature-accelerated deterioration test which has been conventionally carried out at a temperature of from over a hundred ten degrees centigrade to around two hundred degrees centigrade, several months are required to obtain data; if a deterioration test is carried out at a temperature as low as 60.degree. C., at about which the oil-filled electric apparatus is operated, a period of 10 years or more is required. A test requiring such a long period of time is not always preferable from a practical stand point. However, if the data of the low temperature deterioration phenomenon is different than the data extrapolated from the high temperature deterioration test data, the diagnosis of the life of the oil-filled electric apparatus cannot be accurately carried out based on the results thereof. Accordingly, in order to improve the precision of diagnosing the life of the electric apparatus, a long-term low temperature deterioration test still needs to be carried out.